Preparation of white titanium dioxide pigments



Feb. 21,1939. I I w. F. WASHBURN 2,148,283

I I PREPARATION OF WHITE TITANIUM DIOXIDE PIGIIIENTS Filed May 11, 1956 Fe ABSORPTION OF FERROUS AND a FERRIC IRQN IN RELATION TO CONCENTRATION m PIGMENI swam PERCENT OF FERROUS 0R FERRIC SALT RETAINED m FILTER CAKE N (I U I CONCENTRATION OF FERROUS OR FERR C SALT IN PIGMENT SLURRY j E M 5 I l zu 2 I U Fe (50 E If) 4 z 4 3 E GRAMS FERROUS AND FERRIC SALT m o ADSORBED BY IOO GRAMS TiO; 0 o 3 IN RELATION TO CONCENTRATION g 9 OF FERROUS OR FERRIC SALT o g IN THE LIQUID PHASE OF THE 1 PIGM ENT SLURRY E a a I m g 5 o I Fe In 53 I o I I 2 3 CONCENTRATION OF FERROUS OR FERRIC SALT IN LIQUID PHASE CF PIGMENT SLURRY I V WILL/J F. 'VWIDHBUKN/ INVENIOR.

gy mf ge elt ATTORNEY.

1 N E-D; 753M 53 PATENT oF FIcE I PREPARATION OF WHITE TITANIUM DIOXIDE PIGMEN'IS Willis F. Wa'shburn, Metuchen, N. J., assignor, by me'sne assignments, to National Lead Company, New York, N. Y., a-corporatlon of New Jersey Application May, 11. 1936, Serial No. 79,003

2 Claims. (01. 134-58) This invention relates to titanium pigments. desirable in order to obtain pigments oi superior It hasparticular reference to 'a method of treatwhiteness. It is well-known that when the solng hydrous titanium dioxide or mixtures thereof called pigment pulp is subjected to 'a digestion in s with extenders such as barium sulfate and cal- 1 the presence of added mineral, acid and a reducs 5 cium sulfate wherebyai'ter calcination pigments ing agent, the major part of the iron. may be re- 5 of superior whitenessgare obtained. moved. Such treatments involve suspending the Industrial processes for the preparation of tiprecipitated hydrous titanium oxide pigment pulp tanium pigments comprise the thermal hydrolysis in water containing added mineral acid, 'e. g.

of ajtitanium solutiomrusuallyasulfate solution hydrochloric acid and a' reducing agent, e. '8. 'loobtained from a sulfuric acid treatment of a hydrogen sulfide, or a soluble thiosulfate or sultitanium ore and subsequent dissolution of the flde salt. The digestion is preferably conducted resulting digestion cake; The titanium-solutions" at an elevated temperature with agitation-for f found to be most useiulare those obtained fromfrom six to eight hours, sometimes as long as jilmenite, an ore which-contains considerable iron. -twentyefour hours. It will be readily seen that l5- Thus the resulting-titaniumsolution will contain such 'treatments'always included an additional Q a considerable amount of dissolved iron salts. step in the process of manufacturing titanium In order to prevent l'iydrolysis of ferric salts this pigment. This step involves additional expensive iron is kept in areduced state and may even be apparatus; increases the processing time, and A H partially'removedbycrystallization, for example. adds to theiunit p'roductioncosts while, at the a as ferrous sulfate. Thus-,industrial ilmenite sosame time, interrupting .the continuous washing i lutions which are at present used in the manufacprocess. l v

ture of titanium pigments contain a'certaln prog In the prior art, the precipitated hydrous tiportion of iron in the ferrous condition. tanium dioxide is subjected towashlng and filter- As a result of thermal hydrolysis of industrial ing operations prior to subjecting it to the action ilmenite solutions hydrous titanium oxide is preof the reducing agent. In the interval between 25 cipitated. Although there is substantially no the precipitation and reducing agent treatment hydrolysis of iron compounds, the freshly precipithere apparently takes place no oxidation of fertated hydrous titanium oxidealways carries enrous salts to ferric salts.

, trained in the so-called pigment pulp a small By means of ,my present invention the same o amount of iron in the ferrous condition. At the beneficial eflects resulting from the prior art same time the hydrous titanium oxide or pigtreatment may be obtained without the necessity ment pulp always contains a certain percentage of resorting to the expediency of an additional of adsorbed or chemically combined mineral: acid process step. Briefly, my invention comprises depending upon the type of solution employed subjecting hydrous titanium oxide or other tifor ydrolys usually Sulfuric acid. tanium oxide pigment pulp consisting of mixtures 3r In order to remove as much iron as possible so of hydrous tit i oxide ith xt d r that the calcined pigment will possess a satisiacthroughout the regular washing and filtering tory color the crude pigment pulp is subjected treatment to the action of a reducing agent in the prolongidi i i This wash absence of added mineral acid.

ng treatmen s e co ve on y n removing those I iron salts which are not adsorbed by the pigment Thus it be ifi g g g E 3 pulp. Those iron salts which have been taken venflon is an mprove me 0 or o up or adsorbed by the pigment pulp cannot be rej tanium pigments 9f whiteness equgl to pr or moved by washing with water, possibly due to the art Pigments in a manner more emcient' more fact that they all-e oxldizsad f f i to economical, and quicker than prior art methods. 46

. fe ric iron during thewashmg process as will be I have discovered that ferric salts are adsorbed subsequently explained. .Whether this be true or y hydrous t m oxide to a much greater not, the fact is that it is impossible to reduce by gree than ferrous salts and it is another object of mere washing the iron content of a titanium pigy nv t on I0 Prevent the Oxidation of the 50 ment pulp much below 0.1% and the adsorbed or initially adsorbed ferrous iron thus rendering combined mineral acid content, for example, sulunnecessary the use of an additional mineral acid furic acid content, belo'w.5 to 8%. to complete the purification of the pigment.

Although an iron content of about 0.1% did not In order that the object a d advantages of y render the finished pigment unsuited for indusinvention will be readily appreciated, the follow- I? trial use, it was recognized that greater purity was 'ing experiments are of interest in showing the sulfate retained in the cake being-obtained by' difference. In order to obtain approximate comparative data on the amounts of ferrous and ferric sulfate actually adsorbed by the pulp it was assumed that the solid phase of the filter cake had the composition HzTiOa and that the adsorbed or chemically combined sulfuric acid, expressed as S03 was 5%. The proportion of liquid phase in the filter cake and the amount of ferrous or ferric salt dissolved in the-liquid phase, assuming the concentration of the salt to be the same as in the filtrate, were then calculated. This amount was subtracted from the total salt in the filter cake to give the amount adsorbed, and this figure was then expressed in grams adsorbed per 100 grams TiOz. The results follow:

Table No. 1

FeSO Fo|(SO 19a. 01 205. s 197. 4 201. s 205.1 63.1 65. a 1 as. 0 a7. a 67. 7 295. 2 303. 3 298. 0 296. 7 298. 5 irate 131. 4 128. 9 128. 7 11B. 9 122. 6 Filter M 163. 8 174. 4 169. 3 177. 8 175. 9 'liO in cake -per cent.- 38. 54 37. 78 38. 76 38.01 38. 46 Total added saltgrams 3. 436 5. 988 1. 381 2. 628 5. 32 Salt in slurry per cent.. 1. 16 1.97 0. 46 0. 88 l. 78 Salt in filtrate --grams- 1.743 3. 040 0.088 0. 3651 l. 49 Concentration of salt in ii]- trate --per cent 1. 326 2. 359 0. 0684 0. 307 1. 21* Ball; in cake "gains" 1. 693 2. 94.8 1. 293 2. 263 3:83 Conoantrationoisalt cake per cent.. 1. 034 l. 690 0. 764 1. 27 2. 18 Total salt retained in cake per cent" 49. 3 49. 3 93. 0 86.1 72. l Grams salt adsorbed per 100 gr. T101. -'l. 07 1. 50 1.88 2. 96 4.18

. amount of adsorbed iron for both ferrous and ferric iron related to increase in concentration of iron salt in the liquid phase of the pigment slurry. Both figures show graphically that ferric iron as sulfate is more readily adsorbed by by drous titanium oxide than ferrous.

Thus, one of the improved results of my invention, namely, prevention of the formation of ferric iron can be readily understood. By keeping the iron in ferrous condition, it may be more readily removed during the filtering and washing operation.

In order to understand more fully my invention a somewhat detailed description of a typical washing treatment of a precipitated titanium pigment pulp is now given:

After the precipitation of hydrous titanium oxide is complete thehydrolysis solution which now consists of hydrous titanium oxide and acid mother liquor containing dissolved ferrous salts is filtered through a suitablefiltering device. Such a device may be either a rotary vacuum filter or a so-called leaf filter which consists of a multiplicity of frames each carrying suitable filter cloth or paper. The filter cake, that is,

,the titanium pigment pulp is then mixed with the least amount of fresh water necessary to give a smooth fiowing slurry. This procedure is known in the art as repulping. The slurry is.then filtered through a rotary or leaf filter, repulped with additional fresh water, refiltered and repulped for a number of times until the pigment pulp is deemed to be sufficiently washed. It is during this washing process that the iron salts present in pigment pulp are subject to oxidation and adsorption as ferric iron. According to prior art method this filtration and refiltration has to be interrupted to permit the pigment pulp to be subjected to a digestion in water containing a reducing agent and mineral acid in order to remove this adsorbed iron.

According to my invention the reducing agent is brought into contact with the hydrous titanium oxide during the first repulping operation which takes place after the removal of the spent acid hydrolysis liquor. Throughout the entire filtering and washing operation, that is, throughout the several operations of filtering, repulping and refiltering, reducing agent should be kept in. contact with the hydrous titanium oxide. This may be accomplished, for example, by a single addition of a water-insoluble reducing agent, for example, metallic zinc in finely pulverulent form, during the first repulping. When using metallic zinc, sufflcient sulfuric acid will be retained by the pulp to liberate nascent hydrogen through the entire filtering and washing operation. On the other hand additions of water-soluble reducing agents, for example, sulfur dioxide gas and water may be made at each repulping operation. When so conducted, no variation of the regular practice is necessary. The necessity for using added mineral acid is obviated and the filtering and washing operation need not be interrupted by another special reducing treatment. f A wide variety of reducing agents are suitable aim the practice of my invention. Among those which I have found especially useful are metallic zinc, titanous compounds, sulfur dioxide gas. hydrogen sulfide, soluble sulfides, e. g. sodium sulphide. soluble thios'ulfates, e. g. sodium thiosulfate, etc.

The quantity of reducing agent may be varied depending upon the amount of ferrous iron in contact with the precipitated hydrous titanium oxide and the conditions of. the filtering and washing operation. It is not possible to state more specifically a range of concentrations for all reducing agents useful in the practice of my invention. As can be readily determined by those skilled in this art,the quantity of reducing agent employed should be sufiicient to maintain reducing conditions throughout the washing process.

By the term pigment pump used in this description and the claims appended hereto, I mean to include not only precipitated hydrous titanium dioxide but composite pigment mixtures in which hydrous titanium dioxide is intimately associated with extenders such as calcium sulfate and barium sulfate.

I wish also to point out that my invention not only facilitates removal of iron but also other reducible impurities such as traces of other metals originally present in the ore which carry through the process with the titanium, for example, chromium and vanadium.

Titanium pigment pulps obtained from the practice of my invention when subjected to a calcination in the usual manner yield titanium pigments possessing whiteness. brightness and other pigment properties superior to pigment not treated with a reducing agent and at least equal tothose treated according to prior art reducing methods.

The foregoing description of my invention has been given for clearness of understanding and no undue limitations should be deduced therefrom, but the appended claims should be interpreted as broadly as possible in view of the prior art.

I claim: a 1

1., In the manufacture of a white titanium dioxide pigment by means of a process which includes subjecting a titanium pigment pulp containing adsorbed or combined mineral acid as well as oxidizable metallic impurities to a. washing and filtration treatment consisting of a plurality of filtration, repulping and refiltering operations, the step which consists in bringing the said titanium pigment pulp in contact with a water-insoluble reducing agent during the first repulping operation in an amount sufficient to maintain, by reaction with the acid contained in the said pigment pulp, reducing conditions throughout the regular washing and filtration treatment in the absence oi! added mineralacid.

2. In the manufacture of a white titanium dioxide pigment by means of a process which includes subjecting a titanium pigment pulp containing adsorbed or combined mineral acid as well asoxidizable metallic impurities to a washing and filtration treatment consisting of a plurality o1 filtration, repulping and refiltering operations, the step which consists in bringing the said titanium pigment pulp in contact with finely divided metallic zinc during the first repulping operation in an amount suiiicient to maintain, by reaction with the acid contained in the said pigment pulp, reducing conditions throughout the regular washing and filtration treatment in the.

absence of added mineral acid.

WILLIS F. WASHBURN. 

